Assessment of the functional status of platelets for transfusion is confounded by the inherent complexity of the cell, as well as the intricate requirements of sample preparation. A correlation between discoid shape and functional integrity of the platelet has been established. We have developed a simple prototype instrument for measuring the fraction of discoid platelets quantitatively in standard blood bank platelet concentrate units within their bags. Our instrument detects the light scattered (633nm) between 5 and 6 degrees of the forward beam. The plasma may be made to flow perpendicular to the incident beam through a narrow parallel plate gap. Discs orient face on to the beam and consequently scatter light through a smaller angle than the randomly oriented platelet. The change in light scattered at 5 degrees for the flowing (oriented) platelets from that of resting (randomly oriented) platelets is a quantitative measure of the fraction of discoid platelets. This measurement has been correlated with other measures of platelet function. Rigorous light scattering theory has been applied to this problem in order to provide a quantitative method for the wide range of blood bank samples.